This invention relates to electro-optic (EO) devices and, more particularly, to optical modulators on such devices.
Others have recognized that the refractive index of an electro-optic crystal can be varied under the control of an applied electric field. There already are bulk and surface layer effect EO modulators which rely on that principle.
A bulk EO modulator typically comprises a pair of electrodes which are deposited on opposite sides of an electro-optic crystal. In operation, a time dependent modulating voltage is applied across the electrodes to establish a time varying electric field which extends through the crystal in a direction generally perpendicular to the optical axis of the crystal. Thus, when a polarized optical beam is launched into the crystal with its polarization at a predetermined angle relative to the electro-optic axes of the crystal, the beam is phase modulated by the applied field. If the E field of the beam is aligned with the electric or optic axis of the crystal, pure phase modulation is produced. If, however, the beam is launched into the crystal with its E field at an angle relative to the electro-optic axes of the crystal, the beam polarization is altered as a function of the length of the crystal and of the instantaneous intensity of the electric field. Accordingly, an intensity modulated light beam can be obtained by passing the output beam from the crystal through an appropriately polarized filter.
Bulk EO modulators suffer from the disadvantage that the effective intensity of the applied electric field is dependent not only on the amplitude of the modulating voltage, but also on the thickness of the crystal. Thin film bulk EO modulators have been proposed to solve the problem of obtaining a useful response to relatively low modulating voltage levels. Unfortunately, however, the utility of the thin film devices is limited because of the difficulties which are encountered in applying an optical beam thereto without experiencing excessive optical attenuation or unwanted mode conversion (i.e., alteration of the cross sectional shape of the beam).
Surface layer effect EO modulators are promising alternatives to bulk devices for many applications. These modulators usually comprise a pair of interdigital multi-element electrodes which are deposited on the so-called active surface of an electro-optic crystal. In operation, a time dependent modulating voltage is applied across the electrodes to create fringing fields between the adjacent electrode elements. The fringing fields extend into the crystal to a depth which is dependent on the instantaneous amplitude of the modulating voltage and have a periodicity widthwise of the crystal which is dependent on the spacing of the adjacent electrode elements. Hence, when an optical beam is launched into the crystal to interact with the periodic fringing fields, the beam is diffracted.
Heretofore, surface layer effect EO modulators have generally been configured to operate as total internal reflection ("TIR") devices because the amount of optical power which is coupled into each of the diffraction orders of the diffracted beam depends on the length of the effective area of interaction between the input beam and the fringing fields. Thus, the conventional practice has been to launch the input beam into the crystal at an angle selected to cause the beam to make grazing incidence with the active surface of the crystal. Nevertheless, even a TIR EO modulator is insensitive to low level modulating voltages because the input beam has a finite waist diameter in the diffraction limited case.